The equation of the chemical reaction given is:
CH4 + 2O2 --> 2H2O + CO2 + heat
The equation is balanced as the number of atoms of each element are the same on both the sides.
It can be seen that for each mole of CH4, two moles of O2 are required.
The molar mass of CH4 is 16, that of O2 is 32, H2O has a molar mas of 18 and CO2 has a molar mass of 44.
As 16 grams of CH4 are reacting, the number of moles of CH4 is one. For this 2 moles of O2 are required. The mass of 2 moles of O2 is 64 g.
The mass of oxygen required for the given reaction is 64 g.